Timed delayed digital signal producing circuit

ABSTRACT

A time delayed digital circuit includes two inverters used in charging and disclosure a capacitor to produce a precisely delayed output digital signal.

FIELD OF THE INVENTION

The present invention relates to circuits for producing precisely timed delayed digital signals such as square wave signals which can be used for operating charge coupled devices (CCDs).

BACKGROUND OF THE INVENTION

When used for electronic imaging, charged coupled devices referred to as CCDs often require precisely positioned pulses with respect to the horizontal clock signal by using a delayed leading edge. These pulses are produced in response to an input square wave clock signal. For example, a buffer amplifier on a CCD sensor is used to sense the voltages of a floating diffusion. The floating diffusion needs to be reset before the charge is transferred by the next clock cycle. The required pulse, known as a reset pulse, is generated by a precisely delayed leading edge of the square wave signal. Additionally, the output amplifier of the CCD is sampled, held and compared to a charge level on the floating diffusion. The sample and hold also needs precisely positioned pulses. The output amplifier can be controlled also by a timed delayed square wave. In CCD sensors, it is essential that a circuit be provided that has facility of precisely delaying a square wave signal which is used to generate the precisely positioned pulses into required pulsewidth, as well as producing pulses which are precisely positioned and which have a predetermined pulse width.

The classical way of generating such pulses (which are of high frequency in the order of 10-20MHz) is done by means of fixed timed delayed chips and combinatorial logic. This approach is expensive, and very difficult to change the precisely positioned pulses. The required timed delay can also be achieved by resistor capacitor circuits but it is difficult to lengthen delay time and also produce required pulse widths with such circuits.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a circuit which eliminates the above problems and which produces widely variable delayed digital signals such as square waves.

Another object of this invention is to provide an improved circuit for producing a series of widely variable precisely timed delayed pulses.

These objects are achieved in a circuit responsive to an input digital signal for providing an output digital signal, the leading edge of each cycle of the output digital signal being selectively delayed in time from the leading edge of each cycle of the corresponding input digital clock signal, comprising:

(a) first and second inverters connected in series and responsive to the input digital signal for respectively providing complementary output digital signals shifted in phase by 180°;

(b) a first resistor and a capacitor serially coupled between the output of the second inverter and the first inverter; and

(c) threshold means responsive to the voltage at the electrical junction of the first resistor and the capacitor which when one such capacitor charges to produce a voltage above a first level provides one edge of an output digital signal and when the capacitor discharges below a second level provides the other edge of the output digital signal.

Recent advances in digital circuits have permitted them to operate at increasing speeds. Computers including such circuits also run at higher speed and at lower power. The present invention permits high speed digital signals to be precisely time delayed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit in accordance with the present invention which produces a precisely delayed digital signal;

FIG. la shows voltage waveforms at various nodes or points in the circuit of FIG. 1;

FIG. 2 shows a circuit similar to FIG. 1 but which produces a series of precisely time delayed pulses in which the leading and trailing edges are independently controlled, each having a predetermined width;

FIG. 3 shows another circuit in accordance with the invention which produces a series of precisely delayed pulses, with precise pulsewidths; and

FIG. 3a shows voltage waveforms at various nodes or points in the circuit of FIG. 3.

DETAILED DESCRIPTION

Turning now to FIGS. 1 and 1a, there is shown a digital signal delay circuit 10 in accordance with the invention. The circuit 10 includes two serially connected inverters 11 and 12. In this disclosure the input digital signal is a square wave clock signal although other types of digital signals will suggest themselves to those skilled in the art. In response to the input digital signal, inverters 11 and 12 provide complementary square wave signals at nodes (a) and (b) (assuming no or insignificant delay), (see FIG. 1a) which have complementary waveforms that are 180° out of phase. Serially connected at the output of inverter 12 is a resistor 14 and a capacitor 16, the resistance of resistor 14 can selectively be adjusted. An emitter follower amplifier circuit, includes a transistor Q₁ and a resistor 18, with the base electrode of transistor Q₁ being connected at the electrical junction of capacitor 16 and resistor 14. The waveform at node (d) and at node (c) are the same except that, at node (d), the voltage waveform is lowered by base emitter junction drop (0.7 v). Coupled at the electrical junction of the emitter of the transistor Q₁ and the resistor 18 is an inverter circuit 20. This type of inverter is well understood in the art and changes from a high signal level to a low signal level when the voltage at node (d) rises above a selected voltage threshold level (e) and when the voltage at node (d) falls below a selected voltage threshold level (f), the output changes to a high level. See the dotted lines (e) and (f) which intersect the voltage waveform in node (d) in FIG. 1a. The differences between threshold levels (d) and (f) provides a wide variety of controlling time delays of leading and trailing edges. Example of an inverter that can be used is logic gate 74F04 hex inverter manufactured by Fairchild.

In operation when the voltage at node (a) turns high, then the voltage at node (b) is low and the capacitor 16 begins to charge. It should be noted that, at the point in time when node (a) changes from low to high, a spike (g) (FIG. 1a) occurs in the waveforms at node (c) and node (d). This sudden change in voltage level at node (d) is raised almost instantaneously because of the superposition of the voltage on the capacitor (c) with the high voltage from the inverter 11. This enables the capacitor 16 to pump very high initial current through the resistor 14 and provides a longer time for the voltage to exponentially decay. Both the leading and lagging edges of the output square wave at the output of the inverter 20 or node (o) can be adjusted over a longer time interval than has been heretofore been practical.

Turning now to FIG. 2, we see a similar circuit to FIG. 1. Where elements correspond to FIG. 1, the same numerals will be used. A first diode D₁ is connected in series with resistor 14 to the output of inverter 12 and a second diode D₂ and resistor 22 is connected in series across diode D₁ and resistor 14. By means of this arrangement, the leading and trailing edges of the output signal can be adjusted independently. Therefore, inverter 20 can produce a series of pulses. The timing of the falling edge of the pulse is determined by the charging time constant of resistor 14 and capacitor 16 and can be changed by changing the resistance of resistor 14. The timing of the rising edge of each pulse is determined in a similar manner by the discharging time constant resistor 22 and capacitor 16. The circuit shown in FIG. 2 not only precisely delays the leading and trailing edge of each pulse, but also actively provides a desired pulse width.

Turning now to FIGS. 3 and 3a, we see two circuits 30a and 30b. Circuits 30a and 30b are similar in construction with the circuits shown in FIG. 1 with two exceptions. First, the time constants of each circuit are different and second, in circuit 30b at the output of threshold inverter 20b, another inverter 34 is provided. The input to both circuits 30a and 30b is as shown in FIG. 3a, a square wave clock signal. The outputs of the two emitter follower amplifiers are shown. The outputs of the circuits 30a and 30b are shown shifted in time and applied to an AND gate 36. The output of the AND gate 36 is a series of pulses precisely delayed in time and having a desired pulse width which is selected by adjusting the resistance of resistors 14a and 14b of the two circuits 30a and 30b. In this circuit, the position of the resulting pulse at node "0" in FIG. 3 is shown in FIG. 3a. The leading edge 44 of the output AND gate 36 is obviously a function of edge 42 in the output of gate 20b which is in turn controlled by resistor 14b in circuit 30b of FIG. 3. Similarly, the falling edge 46 of the pulse provided by AND gate 36 is a function of edge 40, which in turn is controlled by resistor 14a of circuit 30(a). The width of the resulting pulse is a function of resistor 14b and 14a and the starting position of the pulse is the function of resistor 14b.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be affected within the spirit and scope of the invention. 

What is claimed is:
 1. A circuit responsive to an input digital signal having a leading edge and a trailing edge for providing an output digital signal having a leading edge and trailing edge, the leading edge of each cycle of the output digital signal being selectively delayed in time from the leading edge of each cycle of the corresponding input digital signal, comprising:(a) first and second inverters having respective outputs connected in series and responsive to the input digital signal for respectively providing complementary output signals shifted in phase by 180°; (b) a first resistor and a capacitor serially coupled between the output of the second inverter and the first inverter; and (c) threshold means responsive to a voltage at an electrical junction of the first resistor and the capacitor which when the capacitor charges to produce a voltage above a first level provides one edge of the output digital signal and when the capacitor discharges below a second level provides the other edge of the output digital signal.
 2. The circuit of claim 1 wherein the input and output digital signals are square wave clock signals and the threshold means includes an emitter follower amplifier circuit and a threshold inverter circuit.
 3. A circuit responsive to an input square wave clock signal for providing a series of precisely timed delay pulses, each having a predetermined pulse width, comprising:(a) first and second inverters having respective outputs connected in series and responsive to the input square wave signal for respectively providing complementary output square wave signals having first and second edges shifted in phase by 180°; (b) a first diode, a first resistor and a capacitor serially coupled between the output of the second inverter and the first inverter and a second diode and a second resistor connected in parallel across the first diode and the first resistor with the second diode having an opposite conductivity path than the first diode; and (c) threshold means responsive to a voltage at an electrical junction of the first resistor and the capacitor which the capacitor charges to produce a voltage above a first level provides one edge of an output pulse and when the capacitor discharges below a second level provides an other edge of the output pulse.
 4. A circuit responsive to an input square wave clock signal for providing a series of precisely timed delay pulses each having a predetermined pulse width, comprising:(a) first and second circuits each responsive to the input square wave clock signal, each such circuit including:(i) first and second inverters having respective outputs connected in series and responsive to the input square wave clock signal for respectively providing complementary output signals shifted in phase by 180°; (ii) a first resistor and a capacitor serially coupled between the output of the second inverter and the first inverter; and (iii) threshold means responsive to a voltage at an electrical junction of the first resistor and the capacitor which when one such capacitor charges to produce a voltage above a first level provides one edge of an output square wave signal and when the capacitor discharges below a second level provides an other edge of the output square wave signal, wherein time constants of each circuit are different; (b) means for inverting the output square wave signal from one of the threshold means; and (c) AND gate means responsive to the inverted output square wave signal from one of the threshold means and the output square wave signal from the other threshold means for producing a series of precisely timed delayed pulses, with each pulse having a predetermined pulse width. 